Integrated circuit elements, such as transistors, capacitors and the like, have been drastically reduced in size and increased in density and proximity, which in turn reduce the signal propagation path length and the signal propagation time. However, the material properties and the physical effects by which transistors and other elements function are inevitably compromised as the sizes of integrated circuit elements reduce.
Many improved designs have therefore been provided in order to maintain suitable levels of performance of these elements. For example, lightly doped drain (LDD) structures (generally referred to as extension implants), halo implants and graded impurity profiles have been employed in field effect transistors (FETs) to counteract short channel and punch-through effects and the like. Reduction in device scale has also required operation at reduced voltages to maintain adequate performance without causing damage to the devices, even though operating margins may be reduced.
A principal factor that affects the performance of field effect transistors is the carrier mobility, which determines the amount of current or charge that may flow (as electrons or holes) through a doped semiconductor channel under a specific gate voltage. Reduced carrier mobility in an FET reduces not only the switching speed/skew rate of a given transistor, but also reduces the difference between “on” resistance to “off” resistance. This latter effect increases susceptibility to noise and reduces the number of and/or the speed at which downstream transistor gates can be driven.
It has been shown that mechanical stress in the channel region of an FET can increase or decrease carrier mobility significantly, depending on the stress type (e.g., tensile or compressive stress) and the carrier type (e.g., electron or hole). Typically, tensile stress in the transistor channel region increases channel electron mobility, but decreases channel hole mobility; on the other hand, compressive stress in such a channel region increases channel hole mobility, but decreases channel electron mobility.
In this regard, numerous structures and materials have been proposed for inducing tensile or compressive stress in the FET channel region, such as the use of an underlying SiGe layer for imparting stress from the bottom of the FET channel layer, and/or use of shallow trench isolation (STI) structures, gate spacers, Si3N4 etch-stop layers for imparting longitudinal stress from the sides of the FET channel layer.
However, there are issues, well known to those skilled in the art, regarding the underlying SiGe layer, including formation of dislocation defects that severely impact yield, along with increased manufacturing cost and processing complexity. The STI approach is less costly, but is not self-aligned to the gate and has external resistance (RX) size sensitivity. By using Si3N4 etch-stop layers, on the other hand, gain is limited by the space between two closely placed gates. As transistor scales, the space becomes smaller and thickness of Si3N4 has to be reduced accordingly, resulting in smaller stress effect.
Therefore, there is a continuing need for structures and methods that can provide significantly high stresses for forming high performance FET devices at reduced costs and processing complexity.